1. Field of the Invention
This invention relates to a method for processing an image signal representing an image when a visible image is reproduced from the image signal, and an apparatus for carrying Out the image processing method.
2. Description of the Prior Art
Techniques for reading out a recorded image in order to obtain an image signal, carrying out appropriate image processing on the image signal, and then reproducing a visible image by use of the processed image signal have heretofore been known in various fields. For example, as disclosed in Japanese Patent Publication No. 61(1986)-5193, an X-ray image is recorded on an X-ray film having a small gamma value chosen according to the type of image processing to be carried out, the X-ray image is read out from the X-ray film and converted into an electric signal (image signal), and the image signal is processed and then used for reproducing the X-ray image as a visible image on a photocopy, or the like. In this manner, a visible image having good image quality with high contrast, high sharpness, high graininess, or the like, can be reproduced.
Also, when certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored thereon during its exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318, 4,387,428, and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object, such as the human body. A radiation image of the object is thereby stored on the stimulable phosphor sheet. The stimulable phosphor sheet is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectricaLly detected and converted into an electric image signal. The image signal is then used during the reproduction of the radiation image of the object as a visible image on a recording material such as photographic film, on a display device such as a cathode ray tube CRT) display device, or the like.
Radiation image recording and reproducing systems which use stimulable phosphor sheets are advantageous over conventional radiography using silver halide photographic materials, in that images can be recorded even when the energy intensity of the radiation to which the stimulable phosphor sheet is exposed varies over a wide range. More specifically, since the amount of light which the stimulable phosphor sheet emits when being stimulated varies over a wide range and is proportional to the amount of energy stored thereon during its exposure to the radiation, it is possible to obtain an image having a desirable density regardless of the energy intensity of the radiation to which the stimulable phosphor sheet was exposed. In order to obtain the desired image density, an appropriate read-out gain is set when the emitted light is being detected and converted into an electric signal to be used in the reproduction of a visible image on a recording material, such as photographic film, or on a display device, such as a CRT display device.
In the image recording and reproducing systems, with which image signals are generated and visible images are reproduced from the image signals, the image signals are usually subjected to various types of image processing such that visible images may be obtained which have good image quality and can serve as effective tools in, particularly, the efficient and accurate diagnoses of illnesses. As one of such image processing, gradation processing is known, with which the image density of a reproduced radiation image is adjusted in accordance with the level of the image signal.
Also, frequency processing is carried out on an image signal in order that a visible radiation image having good image quality can be reproduced and used as an effective tool in, particularly, the accurate and efficient diagnosis of an illness. One example of the frequency processing is frequency emphasis processing, such as unsharp mask processing disclosed in, for example, U.S. Pat. Nos. 4,315,318 and 4,317,179. With the frequency emphasis processing, an unsharp mask signal Sus is subtracted from a read-out image signal Sorg, which has been detected from a radiation image. The obtained difference value is multiplied by an emphasis coefficient .beta.. The resulting product is added to the read-out image signal Sorg. In this manner, predetermined spatial frequency components in the image can be emphasized. The frequency emphasis processing is represented by the formula EQU S=Sorg+.beta.(Sorg-Sus) (1)
wherein S represents the signal obtained from the frequency processing, Sorg represents the read-out image signal, Sus represents the unsharp mask signal, and .beta. represents the emphasis coefficient.
Additionally, in order that parts of an image covering as wide a range of image density as possible can be used, the level of contrast of the parts of the image having a high or low image density or the level of contrast of the whole image has heretofore been rendered low such that the difference between the highest image density and the lowest image density may become small, i.e. such that the dynamic range of the image may become narrow. Such processing is disclosed in, for example, U.S. patent application Ser. No. 646,123, now abandoned.
Further, when a radiation image (i.e., a tomographic image) of a tomographic plane is recorded with a tomographic image recording operation described in, for example, U.S. Pat. No. 4,581,535, it often occurs that the amount of transmitted radiation changes sharply at a portion, which is located at a position other than the tomographic plane. In such cases, an image pattern of such a portion occurs as an interfering pattern (or an interfering shadow) in the direction, along which the recording medium moves, and at a center region which is to be used in the radiation image. (Such an interfering pattern will hereinafter be referred to as the "flow pattern.") An image processing method for eliminating the flow pattern is proposed in, for example, Japanese Unexamined Patent Publication No. 3(1991)-276265. With the proposed image processing method, low spatial frequency components corresponding to the flow pattern are removed from the image signal representing the radiation image obtained from the tomographic image recording operation, and an image free of the flow pattern is thereby obtained from the resulting image signal.
Various types of the image processing described above, such as the gradation processing and the frequency processing, are respectively carried out with image processing apparatuses, in which the calculation processes suitable for the respective image processing are stored.
However, if different image processing apparatuses are provided for a plurality of the types of the image processing, the problems will occur in that the calculation processes and the image processing apparatuses cannot be kept simple, and in that the number of the image processing apparatuses cannot be kept small.